Poo, or pooh, is a perfectly acceptable interjection expressing contempt. It dates from the late 16th Century. In the last decade, though, the word poo has come to be used almost exclusively as a synonym for defecation. As such it is priggish nursery-talk and to the nursery it should be banished. I blame all this pooing on those women journalists who write weekly columns in the national press. Their angst-ridden outpourings, usually dreamt up in bespoke home offices on the mean streets of Notting bloody Hill, are concerned mostly with the horrors of soiled nappies and with the unreliability of Bulgarian au pairs. Thanks to these hacks and their puritanical papers, the steady proliferation of the piss-poor poo word has continued unabated.
Two perfectly good colloquialisms meaning faeces spring immediately to mind. We borrowed 'shit' from one of the old Teutonic languages a good 500 years ago and 'crap' has been appearing in print since 1780 and probably earlier. Consult Mr Roget (he was English-born and the remoteness of his Swiss ancestry makes a "Monsieur" unnecessary) and you will find several other synonyms but no 'poo'.
Let's try to stop using the prudishly prissy 'poo' and revert to good old Old English.